The decision by ratings agency Standard & Poor's to downgrade the debt of the world's largest economy has sparked a ferocious debate in America.

Politicians and pundits have piled into a gigantic blame game of who should carry the can for the decision, which has unnerved investors worldwide ahead of stock markets opening tomorrow.

Critics of the ratings agency's power have pointed out flaws in Standard & Poor's sums and reasoning, arguing that far too much weight is given to the opinions of one private company. Defenders say that the firm is only acknowledging what many other pundits have said for a long time: the US has too much debt and its fractious political system seems unable to solve it.

Leading the charge of critics is Nobel prizewinning economist and New York Times columnist Paul Krugman. In a blog post he took Standard & Poor's calculations to task and mocked it and other agencies for helping to inflate the recently burst housing bubble with poorly rated securities. "It's hard to think of anyone less qualified to pass judgment on America than the rating agencies. The people who rated subprime-backed securities are now declaring that they are the judges of fiscal policy? Really?" Krugman fumed.

Meanwhile, Republican contenders for the right to fight Obama in 2012 ignored the ratings agencies and instead blasted the president. "America's creditworthiness just became the latest casualty in President Obama's failed record of leadership on the economy," said former Massachusetts governor Mitt Romney. "This happened on the president's watch – and he has to deal with it," said conservative former senator Rick Santorum.

Some experts were baffled by the potential implications of the downgrade. Washington Post columnist Dylan Matthews wrote that the downgrade could force some institutions to sell US treasury bonds as they are legally bound to maintain a certain amount of their investments at AAA, not the US's current AA+.

The downgrade could also force up interest rates, which could damage the already weak US economy. But the downgrade might not matter at all, Matthews added. The debt ceiling has been lifted, AA+ is still an excellent rating and few seriously believe the US will default anytime soon. "The United States has never been in this situation before, so it is hard to see who is correct. There is simply no modern precedent for a US downgrade," Matthews wrote.

About the only people not making much of a statement about the downgrade were the White House. Obama, who was weekending at the presidential retreat of Camp David, has not made any official pronouncement.

Meanwhile, presidential spokesman Jay Carney appeared to address Standard & Poor's concerns that American politics was too divided to produce useful economic leadership in tackling the size of the nation's debts.

In a statement Carney said the path to a recent deal to raise the US's debt ceiling had been "too long … too divisive" and urged all politicians to avoid a repeat of the bitter battle. "We must do better to make clear our nation's will, capacity and commitment to work together to tackle our major fiscal and economic challenges," he said.